1. Field of the Invention
The present invention relates to bi-directional wavelength-division-multiplexing (WDM) optical communications and, more particularly, to an optical amplifier device used in the bi-directional WDM optical system.
2. Description of the Related Art
A WDM optical communication system is adapted to transmit a plurality of channels through an optical fiber. As such, this type of system is widely used in the ultra-high-speed Internet as it has superior characteristics in terms of transmission efficiency and transmission capacity. As optical signals transmitted along the optical fiber tend to attenuate in proportion to the transmission length thereof, an optical amplifier is installed along the optical fiber for amplification of the attenuated channels.
FIG. 1 is a circuit diagram illustrating a conventional bi-directional WDM optical communication system. As shown in FIG. 1, the system includes a first optical transmitter/receiver unit 100, an optical amplifier unit 200, a second optical transmitter/receiver unit 300, and an optical fiber 400. The optical signal transmitted from the first optical transmitter/receiver unit 100 to the second optical transmitter/receiver unit 300 is referred to as a forward optical signal 115, whereas the optical signal reversely transmitted is referred to as a reverse optical signal 315. Each of the forward and reverse optical signals 115 and 315 is composed of a plurality of channels with different wavelengths.
The first optical transmitter/receiver unit 100 includes a plurality of first transmitters 110, a plurality of first receivers 180, a first wavelength-division multiplexer 120, a first wavelength-division demultiplexer 170, a first dispersion-compensation fiber 130, a first amplifier 140, a second amplifier 160, and a first optical circulator 150. In usage, the first transmitters 110 output channels of different wavelengths, respectively. The first wavelength-division multiplexer 120 receives the channels from the first transmitters 110, and multiplexes the received channels, thereby outputting a channel-multiplexed signal, that is, the forward optical signal 115. The first dispersion-compensation fiber 130 compensates the dispersion in the forward optical signal 115. The dispersion of the forward optical signal 115 is caused by the fact that the channels of the forward optical signal 115 have different wavelengths, respectively. The first amplifier 140 amplifies the forward optical signal 115 by utilizing an induced emission of erbium ions and a laser diode for emitting pumping light adapted to excite the erbium ions, and a wavelength-selection coupler for applying the pumping light to the erbium-doped optical fiber. Thereafter, the first circulator 150 transmits the forward optical signal 115 received from the first amplifier 140 to the optical amplifier unit 200 via the optical fiber 400, while sending the reverse optical signal 315 from the optical amplifier unit 200 to the second amplifier 160.
The second amplifier 160 amplifies the reverse optical signal 315 inputted thereto and applies the amplified reverse optical signal 315 to the first wavelength-division demultiplexer 170. Then, the first wavelength-division demultiplexer 170 demultiplexes the reverse optical signal 315 into a plurality of channels with different wavelengths. The first receivers 180 receive the channels outputted from the first wavelength-division demultiplexer 170. Meanwhile, the optical fiber 400 couples the first and second optical transmitter/receiver units 100 and 300 to each other, and serves as a transmission medium for the forward and reverse optical signals 115 and 315.
Next, the optical amplifier unit 200 includes a second circulator 210, a third circulator 240, a second dispersion compensation fiber 220, a third dispersion compensation fiber 250, a third amplifier 230, and a fourth amplifier 260. In operation, the second circulator 210 transmits the forward optical signal 115, received from the first optical transmitter/receiver unit 100 via the optical fiber 400, to the second dispersion compensation optical fiber 220, while sending the reverse optical signal 315 from the fourth amplifier 260 to the first optical transmitter/receiver 100 via the optical fiber 400. The second dispersion compensation optical fiber 220 compensates the dispersion of the forward optical signal 115 and applies the dispersion-compensated forward optical signal 115 to the third amplifier 230. The forward optical signal 115 is amplified by the third amplifier 230, and then applied to the third circulator 240. The third circulator 240 transmits the forward optical signal 115, inputted thereto, to the second optical transmitter/receiver unit 300 via the optical fiber 400, while sending the reverse optical signal 315, inputted thereto via the optical fiber 400, to the third dispersion-compensation optical fiber 250. The third dispersion-compensation optical fiber 250 compensates for a dispersion of the reverse optical signal 315. The fourth amplifier 260 amplifies the dispersion-compensated reverse optical signal 315, and then transmits the amplified reverse optical signal 315 to the second circulator 210.
The second optical transmitter/receiver unit 300 includes a plurality of second transmitters 310, a plurality of second receivers 380, a second wavelength-division multiplexer 320, a second wavelength-division demultiplexer 370, a fourth dispersion-compensation fiber 330, a fifth amplifier 340, a sixth amplifier 360, and a fourth circulator 350. In use, the second transmitters 310 output channels of different wavelengths, respectively. The second wavelength-division multiplexer 320 receives the channels from the second transmitters 310 and multiplexes the received channels, thereby outputting a channel-multiplexed signal, that is, the reverse optical signal 315. The second dispersion-compensation fiber 330 compensates the dispersion of the reverse optical signal 315. The fifth amplifier 340 amplifies the reverse optical signal 315. The fourth circulator 350 transmits the reverse optical signal 315 received from the fifth amplifier 340 to the optical-amplifier unit 200 via the optical fiber 400, while sending the forward optical signal 115, received from the optical-amplifier unit 200 via the optical fiber 400, to the sixth amplifier 360. The sixth amplifier 360 amplifies the forward optical signal 115 inputted thereto, and applies the amplified forward optical signal 115 to the second wavelength-division demultiplexer 370. The second wavelength-division demultiplexer 370 demultiplexes the forward optical signal 115 into a plurality of channels with different wavelengths. The second receivers 380 receive the channels outputted from the second wavelength-division demultiplexer 370, respectively.
FIG. 2 is a graph depicting the forward and reverse optical signals 115 and 315. As shown in FIG. 2, the bi-direcional WDM optical communication system of FIG. 1 uses a wavelength band ranging from 1,532 nm to 1,554 nm. Readers can note that the wavelength space between adjacent channels in the forward (or reverse optical signal) 115 (or 315) is 2 nm. The wavelength band of the forward optical signal 115 ranges from 1,532 nm to 1,542 nm, whereas the wavelength band of the reverse optical signal 315 ranges from 1,544 nm to 1,554 nm. That is, the forward optical signal 115 is distributed within a relatively short-wavelength band, whereas the reverse optical signal 315 is distributed within a relatively long-wavelength band. Such an multi-channel optical signal is subjected to signal distortion due to a cross phase modulation (XPM) phenomenon caused by a signal-power difference between adjacent channels, a four-wave mixing (FWM) phenomenon, i.e., the introduction of noise from a channel into adjacent channels, the dispersion phenomenon, and the scattering phenomenon. Where an optical signal is severely distorted, it cannot be received by the receiver unit. For this reason, the wavelength space of adjacent channels is set, taking into consideration such a signal distortion of optical signals. This wavelength space is referred to as a “minimum wavelength space.” The available wavelength band of an optical signal is limited due to an attenuation of the optical signal caused by loss characteristics of the optical fiber through which the optical signal passes. The available wavelength band is referred to as a “maximum wavelength band.” As a result, the maximum wavelength band in the bi-directional WDM optical communication system shown in FIG. 1 is 1,532 nm to 1,554 nm. Accordingly, the maximum number of channels in an optical signal transmittable at a minimum wavelength space of 2 nm is 12.
FIG. 3 is a circuit diagram illustrating the optical amplifier unit 200 shown in FIG. 1. As shown in FIG. 3, the optical amplifier unit 200, which includes the second circulator 210, third circulator 240, second dispersion compensation fiber 220, third dispersion compensation fiber 250, third amplifier 230, and fourth amplifier 260, further includes a first isolator 232, a second isolator 262, a first erbium-doped optical fiber 234, a second erbium-doped optical fiber 264, a first wavelength-selecting coupler 238, a second wavelength-selecting coupler 268, a first laser diode 236, and a second laser diode 266. The third and fourth amplifiers 230 and 260 have the same configurations as those of the first and second amplifiers 140 and 160 or those of the fifth and sixth amplifiers 340 and 360, respectively. For simplicity, the duplicated configurations of the amplifiers will be omitted herein.
The first isolator 232 transmits the forward optical signal 115, received from the second dispersion-compensation fiber 220, therethrough while preventing optical signals, traveling in a reverse direction of the forward optical signal 115, from passing therethrough. For this reason, the pumping light 237 traveling along the first erbium-doped optical fiber 234 after being outputted from the first laser diode 236 cannot pass through the first isolator 232. Thereafter, the first erbium-doped optical fiber 234 amplifies the forward optical signal 115 by utilizing an induced emission of excited erbium ions. At the same time, the pumping light 237 is emitted from the first laser diode 236 for exciting erbium ions. The first wavelength-selecting coupler 238 transmits the amplified-forward optical signal 115 therethrough, so that the amplified-forward optical signal 115 is applied to the third circulator 240. The first wavelength-selecting coupler 238 also sends the pumping light 237, received from the first laser diode 236, to the first erbium-doped optical fiber 234.
Similarly, the second isolator 262 transmits the reverse optical signal 315, received from the third dispersion-compensation fiber 250, therethrough while preventing optical signals, traveling in a reverse direction from the reverse optical signal 315, from passing therethrough. For this reason, the pumping light 267 traveling along the second erbium-doped optical fiber 264 after being outputted from the second laser diode 266 cannot pass through the second isolator 232. The second erbium-doped optical fiber 264 amplifies the reverse optical signal 315. The pumping light 267 is emitted from the second laser diode 266. The second wavelength-selecting coupler 268 transmits the amplified reverse optical signal 315 therethrough, so that the amplified reverse optical signal 315 is applied to the second circulator 210. The second wavelength-selecting coupler 268 also sends the pumping light 267, received from the second laser diode 266, to the second erbium-doped optical fiber 264.
Although not shown, it is necessary to additionally provide a laser diode driving unit at each of the optical fiber amplifiers 230 and 260 in order to supply a drive current to an associated one of the laser diodes 236 and 266. It may be also necessary to additionally provide a gain-flattening filter in order to compensate for the gain unbalance caused by non-uniformity in the amplification rate of each erbium-doped optical fiber 234 or 264 resulting from a variation in the wavelength of the input optical signal.
As mentioned above, the conventional optical amplifier device has respective configurations adapted to amplify forward and reverse optical signals. For this reason, the conventional optical amplifier device has drawbacks in that it requires duplicated elements, i.e., optical amplifiers. In addition, it is necessary to use multiple-dispersion-compensation fibers and other optical elements. For this reason, the conventional optical amplifier device increases manufacturing costs and maintenance costs. Furthermore, the conventional bi-directional WDM communication system is configured to divide their available wavelength band into long and short-wavelength bands to be respectively allocated to forward and reverse optical signals. For this reason, there is a problem in that the channel density in the maximum wavelength band is reduced.